In fibre optical transmission systems there occurs the problem of being able to switch the signals passing through one or more optical fibres to another optical fibre or fibres with the least possible attenuation. Analogous with conventional signal transmission fibre-optical relays have therefore been designed to provide for this type of switching. Fibre-optical relays are known in the art, one such being described in the British Pat. No. 1,580,109. This relay has a substrate which is formed with two grooves for clamping two optical fibre ends, and with a cavity in which one of the fibres extends so that its end portion can move laterally to assume two different positions under the action of a magnetic field. In one position the movable fibre end is optically coupled to the fixedly arranged fibre end, while in the other position the fibre ends are disconnected. A third fibre end may possibly be fixed such that in the second position it is connected to the first movable fibre end. Switching between two optical signal parts may thus take place.